This invention relates to a multi-layered transducer and method of manufacturing the transducer. For example, a multi-layered, multi-dimensional transducer is used. Multi-dimensional transducer arrays include 1.5-dimensional (1.5D) and 2-dimensional arrays. For example, an array of N×M elements where both N and M are 2 or greater is provided for ultrasonically scanning a patient. 1.5D arrays typically comprise arrays of 64 or 128 azimuthally spaced elements in each of three, five or more elevationally spaced rows.
Multi-dimensional transducer arrays typically have small plate areas or areas for transmitting acoustic energy from the azimuth and elevational plane. Multiple layers account for the small plate areas. The multiple layers are stacked along the range dimension. Multiple layers for each element reduce the electrical impedance when compared to an equivalent element of only one layer. The capacitance of a transducer element increases by the square of the number of layers forming the transducer element. The increased capacitance of the transducer element results in a decrease of the electrical impedance of the transducer element.
In one method of fabricating a multi-layer transducer assembly, sheets of piezoelectric ceramic are formed from raw materials by tape casting. An internal electrode is screen-printed on a sheet of piezoelectric ceramic, and then another sheet of ceramic is laminated on the internal electrode side of the first sheet. External electrodes are printed and fired on the external sides of the first and second sheets. For example, Saithoh, S. et al., “A Dual Frequency Ultrasonic Probe,” Jpn. J. Appl. Phys., vol. 31, suppl. 31-1, pp. 172-74 (1992), describes such a method. The signal electrodes are connected to leads using a flex circuit, TAB-like jumpers or wire bonding. The ground electrode is connected using a conductive epoxy that contacts the ground electrode and a secondary connector, such as a flex circuit or a metal foil.
Multi-layer transducers are also fabricated with vias to connect similarly oriented layers. Multiple holes are punched mechanically or by laser, drilled or etched into piezoelectric ceramic tape to form the vias on each layer of piezoelectric ceramic. The via holes are filled with a metal paste, and the surface electrodes for each layer are deposited by screen printing. Multiple layers of green tape are then superimposed to align the vias to form a multi-layer sandwich. The multi-layer sandwich is laminated and sintered to form a single structure. Electrodes are metallized by plating or vacuum deposition on the input pads. For an example of such a process, see U.S. Pat. No. 5,548,564, the disclosure of which is incorporated herein by reference.